Polyoxymethylene polymers, those having recurring —CH2O— units, have been known for many years. These polymers may be prepared by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde. Also known is the preparation of oxymethylene copolymers by copolymerizing trioxane with cyclic ethers such as dioxolane. These polymers and copolymers are used as molding resins. See U.S. Pat. No. 3,027,352, as well as “Acetal Resins”, by T. J. Dolce and J. A. Grates, Second Edition Of Encyclopedia Of Polymer Science And Engineering, John Wiley And Sons, New York, 1985, Vol. 1 pp. 42–61.
Present state of the art includes means to impart desired properties to oxymethylene based molding compositions by inclusion of certain additives in order to maintain the workability and stability of the polymers. U.S. Pat. No. 5,173,532 discloses the use of calcium carbonate in polyoxymethylene compositions. U.S. Pat. No. 5,886,066 teaches use of at least about 0.25 weight percent of calcium carbonate to improve surface wear resistance. See, generally, U.S. Pat. Nos. 4,996,253; 4,517,319; 5,354,798; 6,284,828; and 4,996,253. As to increasing stability and impact strength see U.S. Pat. Nos. 3,699,062; 4,731,396; 4,780,498; 4,886,851; 6,156,834; and 4,831,073. As to increasing platability and color stability see U.S. Pat. Nos. 5,096,951; 5,094,713; 4,418,162; and 4,836,899. As to increasing thermal stability see U.S. Pat. Nos. 4,837,400; 5,948,844; 5,212,222; 5,939,481; 5,478,895; and 4,861,814. As to anti-static properties see U.S. Pat. Nos. 5,886,066; 4,647,609; 5,106,896; 5,641,824; 4,559,380; 4,649,179; and 4,274,986. As to improving frictional properties, see U.S. Pat. No. 5,482,987. As a carrier for anti-bacterial and anti-fungal agents see U.S. Pat. No. 5,478,563. As to decreasing porosity see U.S. Pat. No. 5,298,537. A recurring problem characteristic of polyoxymethylene molding resins is their susceptibility to degradation such as delamination by bleaching agents. Despite all of the advances made in the polyoxymethylene injection molding field, there is a need for improving bleach resistance of resin compositions and parts prepared therefrom.